CD59 is a membrane-bound glycoprotein that inhibits the formation of the complement membrane attack complex (MAC or C5b-9) on host cells. CD59 functions by binding to C8alpha and C9 in the assembling MAC and interfering with its membrane insertion. Inappropriate complement activation and MAC deposition is involved in the pathogenesis of many autoimmune and inflammatory diseases, and is involved in hyperacute rejection of xenografts. Understanding the molecular interaction between CD59 and its ligands will provide a foundation for designing modified and improved complement inhibitors. In addition, CD59 expressed on tumor cells has been implicated in promoting tumorigenesis. The accomplishment of aims will enable design of small molecules to effectively block the function of CD59 on tumor cells. The goal of the application is to define the molecular interaction between CD59 and its C8 and C9 complement protein ligands. Specifically it is proposed to: 1. Identify the specific CD59 residues that are important for function. This aim will be accomplished using systematic scanning mutagenesis as well as rational substitutions of important regions and residues, with assistance from molecular modeling. 2.ldentify CD59 residues directly involved in ligand binding to determine whether C8 and C9 share the same binding residues on CD59, and to determine the nature of the species selective interaction between CD59 and its ligands. 3. Identify the C8 and C9 residues that bind to CD59 by analysis of effects of mutagenesis within identified C8/C9 binding regions. 4. Determine the three-dimensional structure of the CD59-C8 and CD59-C9 peptide ligand. Molecular modeling techniques together with experimental determinations by NMR will be utilized.